Stroppy Child Syndrome!

Hi all, advice needed!
My youngest is 4 and I've been told by the pre-school that his strops are getting more frequent and they have no idea how to handle it, they have tried the ignore until it's over routine, but frankly that doesn't bother him! He will strop at the smallest of things and crosses his arms with his bottom lip firmly prominent! I have tried all sorts and patience is wearing thin! Any suggestions?

Bless you, hugs appreciated! He can be very charming and I think I have to hold my hand up and admit I tend to let him get away with some minor things as he's the youngest! But I shall perceiver and hopefully get it sorted before going to school! xx